One of the most effective materials in current use for the direct conversion of x-ray energy into an electrical signal is Cadmium Zinc Telluride (CZT). Electrodes of CZT radiation detectors are presently formed of platinum thin films. The platinum forming each electrode is deposited via a suitable thin film deposition technique such as, without limitation, sputtering, to a thickness of approximately 1000 Angstroms. However, this thickness is not to be construed in a limiting sense.
Each platinum electrode defines an electronic contact that enables the CZT bulk material to have superior performance when acting as an x-ray or gamma-ray radiation detector and to interface with electronics with sufficient connectivity to allow the data acquisition and control electronics to form a functional and high performance CZT detector based sensor.
In the case of a CZT detector based sensor system, platinum electrodes provide for the proper work function and chemical bond to the CZT bulk material so that the work function matches the bandgap, resistivity, and interface electronic properties of the CZT bulk material, thereby enabling the CZT bulk material, or other suitable compound semiconductor crystal material, to realize full depletion its entire volume with excellent collection of charge carriers generated by the absorbed x-rays and gamma rays. The high sensitivity provided by the full depletion of the entire CZT bulk material volume and the good charge collection ensures nearly loss-free signal generation from absorbed photons and provides higher performance levels of CZT based detectors than do many other semiconductor detectors.
In the manufacture of compound semiconductor devices, such as CZT detectors, the mechanical fragility of the delicate thin platinum electrode metal prohibits the use of robust manufacture methods to fabricate these devices. Specifically, after deposition, the delicate thin-film platinum electrodes are easily damaged during subsequent device fabrication steps such as: in-line probe testing; device handling during downstream device processing; and bonding to interconnect substrates or read-out electronics during integration to signal processing electronics.
After deposition on a CZT detector, platinum electrodes are soft and frequently scratched during subsequent handling and manufacture of CZT radiation detectors. Even minor scratching of these delicate platinum electrodes can cause damage to the underlying CZT-to-metal interface and can generate electronic noise in the CZT detector. At a minimum, this damage can deteriorate the performance and reduce the operative range (bias voltage, temperature) of the CZT detector. Often this damage is catastrophic and renders the CZT detector unusable for its intended application. When scratched, a CZT detector may require rework, re-metallization, or may be lost as scrap.
This crystalline material is extremely fragile. When electrical connections to the crystal are made using packaging techniques commonly applied to semiconductors, a significant shift in energy response ratios can result due to increased resistance in the interconnection between the CZT and the data acquisition and control electronics. A possible resolution of this problem is to use a low temperature, reflowed solder connection, although the solder melting/joining process may diffuse or dissolve the thin CZT electrical connection pads and thereby cause connection problems, which adversely affect the performance of the crystal.
The solder melting/joining that can diffuse or dissolve the thin CZT electrical connection pad causing crystal interconnect degradation is the problem solved by the current invention. Important features of a typical crystal detector package are used herein to describe the art as it is currently practiced. A detector crystal is attached to a substrate via a series of electrically conductive interconnections to form a detector package. In some instances an underfill material is disposed between the crystal and the substrate. Electrical traces within the substrate are in electrical communication with the electronics used to amplify and process signals generated within the crystal.
A combination of multiple heat applications and possible residual solder flux materials remaining on the attachment substrate pads contributes to degrade the contact pads located on the CZT crystal, thereby causing attachment and electrical connection issues. The resultant sub-optimal connection at this interface often produces a shift in energy response ratios by the crystal structure.
Whatever the source of degradation, the spectral performance of the crystal is degraded at low photon flux, and the linearity of the system is diminished at higher flux levels. Connection degradation must be reduced or preferably eliminated from x-ray detector crystal signal interconnect pathways. An interface is desired between the attachment substrate and the contact pads of a CZT crystal package that is mechanically robust and has low electrical resistance.